Allow Db2 SysAdmins to specify explicit numbers of DIR or DATA entries, as an alternative to RATIO

The Challenge

The current mechanism for specifying the number of Directory Entries and/or Data Pages in a Group Bufferpool is the RATIO parameter. This worked well enough when memory sizes were small.

With Db2 for z/OS bufferpools in the 10s and even 100s of GB, IBM Distinguished Engineers have recommended larger DIR space for the Group Bufferpools, to allow data-sharing to keep track of larger numbers of pages in the local pools and avoid XI for directory reclaim. As a result, RATIO numbers have trended much larger, to the point where IBM has had to implement APAR PH13045 to increase the max RATIO from 255 to 1024.

The RATIO mechanism itself, however, is "klunky" at best. Since DIR and DATA entries are different sizes, complicated calculations are often necessary to determine the correct RATIO to yield a certain number of one type of entry, and how much that would increase/reduce the other type of entry. Db2 provides an "estimate" of number of DIR and DATA entries when -ALT GBPOOL RATIO is issued, but this estimate is, and has historically been, wrong.

The Potential Solution

Therefore, instead of relying on a RATIO to specify the split between DIR and DATA, I am suggesting that Db2 z/OS SysAdmins be allowed to explicitly specify the number of DIR entries or the number of DATA entries on the -ALT GBPOOL command.

I am not sure how the CF actually splits the group bufferpool...whether Db2 passes XCF/XES a "RATIO" value or if it passes it explicit numbers for memory size/number of entries of DIR or DATA...but it would benefit us if the human had a simpler/more straightforward interface, and the computer did what it was best at...the math.